A speaker or writer uses my to indicate that something belongs or relates to himself or herself.
I invited him back to my flat for coffee.
我请他回我的公寓喝咖啡。
In conversations or in letters, my is used in front of a word like 'dear' or 'darling' to show affection.
My sweet Freda.
我亲爱的弗雷达。
My is used in phrases such as 'My God' and 'My goodness' to express surprise or shock.
My God, I've never seen you so nervous.
我的天呀!我从没见你这么紧张。